For example, as a rotational angle detection device for detecting the rotational angle by means of a conventional resolver, there is a rotational angle detection device, which is described in Japanese Patent Application No. 2002-127173 (hereafter referred to as “prior art”). As shown in FIG. 5, prior art describes that in case that an exciting coil 122, a cos phase coil 128, and a sin phase coil 130 are grounded through a common earth wire 146, the voltage which is made as a result that an alternating current bias voltage which is caused by an impedance 144 of the earth wire 146 and an exciting current, is superimposed on an alternating-current rotational angle voltage whose amplitude fluctuates in dependence on a sin value of a rotor rotational angle θ is output to a terminal 136 of the sin phase coil 130. As a result, a problem arises in that the detection accuracy of a rotor 122 rotational angle, which is gained from a voltage output from the terminal 136 of the sin phase coil 130, is lowered. To solve this problem, the prior art discloses the following rotational angle detection device. First, a steering wheel is slowly rotated through one revolution, during which the sin phase voltages are sampled to be stored in a RAM. Then, from the stored data in the RAM, four points of the one-cycle sin phase voltages are selected for data around the maximum peak value (at a 90-degree steering wheel angle) and around the maximum bottom value (at a 270-degree steering wheel angle), and then, those data are added in order. By dividing each value of these added data by numeral 2, only the bias voltage can be derived. By subtracting this bias voltage from the sin phase voltage, a rotational angle voltage is calculated. And then, a rotational angle is derived from this rotational angle voltage.
However, in the prior art, sufficient consideration is not given regarding a temperature variation which varies every moment. For example, a problem remains unsolved in that the rotational angle cannot be detected accurately for the following reason. That is, a resistance value of the impedance 144 has a temperature drift due to a variation in the ambient temperature, and as a result, the output voltage from the terminal 136 of the sin phase coil 130 also has a temperature drift.
Accordingly, the present invention has been made aiming at the above unsolved problem in the prior art, and it is an object of the invention to provide a rotational angle detection device which is highly accurate not to involve the rotational angle error due to a temperature variation which varies every moment.